Infragravity Waves: Part 3

Infragravity Waves: Part 3

In the last two articles I’ve been talking
about infragravity waves, what they are, where they come from and how they can
sometimes completely dominate the water motions at the shoreline. In this, the
final part, I’m going to talk about why infragravity waves are perhaps the most
important link between oceanic storms and coastal erosion; plus I’m going to
say a bit more about what I mentioned right at the very beginning – how
infragravity waves can penetrate right through to the most far-reaching
pocket-beach halfway up some estuary or behind several breakwaters, even when
the ordinary waves have been whittled down to a fraction of their open-ocean
size.

For many years, people have known that
beaches tend to erode in large storms. The classic pattern is a long, slow
phase of onshore sediment movement during periods of small waves, followed by a
short, powerful burst of offshore sediment movement during an episodic storm
event. But how this pattern actually
takes place is a bit more of a mystery. What is the process that links big
storms with coastline erosion? We still don’t know the complete answer, but
infragravity waves go some way towards explaining it.

The standard theory linking storms with
coastal erosion through infragravity waves is as follows. Under the crest of a
wave, including that of an infragravity wave, the water motions are in the same
direction as the wave itself. Under the trough, the water motions are in the
opposite direction. Therefore, under the trough of a shoreward-travelling
infragravity wave, the water motion is offshore (Figure 1).

Figure
1: The water motions beneath a passing wave

The ordinary waves have underwater motions
associated with them too. These water motions go shoreward and seaward much
faster than those of the infragravity waves, and stir up sediment on the bed.
But the amount of sediment stirred up depends on where you are in the wave
group, or set (waves travel in sets in deep water, way before they reach the
breakpoint – sometimes you can see them from the top of a cliff). Assuming that
the waves get bigger towards the middle of the group and taper off towards the
beginning and end, we can say that the large waves in the middle of the group
produce the most powerful water motions. Therefore, the most sediment is
stirred up under the middle of the group. As a result, you have areas of high
sediment suspension under the middle of the group, and areas of little or no
sediment suspension under the beginning and end of the group.

Now, according to the most accepted theory
of infragravity-wave formation, which I briefly explained in the last article,
the trough of the infragravity wave coincides with the middle of the wave
group. So the trough of the infragravity wave, under which there is an offshore
flow of water, coincides with the large ordinary waves in the middle of the group,
under which there are areas of high suspended sediment concentration. This,
logically, will transport large amounts of sediment offshore. In contrast, the
peak of the infragravity wave, where the water motion is onshore, coincides
with much smaller ordinary waves in between groups, under which there are areas
of little or no sediment suspension. Hence there will be only small amounts of
sediment transported onshore.The whole
story is shown in Figure 2.

In summary, if you add up the onshore and
offshore sediment transport in stormy conditions when you have large
infragravity waves, you will find a net offshore transport.This contributes to coastal erosion.

Of course, the theory above will only work
if at least some of the infragravity waves are still locked to the groups. But
if they are no longer locked to the group after the breakpoint, surely they can
only produce coastal erosion beyond
the breakpoint, right? If that were the case, infragravity waves wouldn’t be a
very convincing explanation of coastal erosion.After all, much more coastal erosion takes place shoreward of the
breakpoint than seaward of it.

In fact, what happens in practice is that
the grouping structure is not entirely
lost at the breakpoint; some of the infragravity waves are still at least
partly attached to the groups. So we can say that the above process can explain
some of the erosion that takes place
shoreward of the breakpoint.

Right on the shoreline, there is also
another process that contributes to sediment erosion. It is in the swash zone where the majority of coastal
erosion happens during storms. The swash zone is the part of the beach right at
the shoreline where the water surges in and out. And the particular nature of
the infragravity motions in the swash zone is one of the most important
factors. It is thought that, when the infragravity waves get really big, they
take on an ‘asymmetric’ form, where the shoreward surge is long and slow, and
the seaward return is short and fast. If you watch carefully in really stormy
conditions you can see the infragravity wave gradually pumping shoreward, with
the ordinary waves ‘stacked up’ on the back of it, before the whole thing slows
down, turns around, gathers itself up and gets sucked out to sea again. Because
the onshore phase is slow, relatively little sediment is churned up.So, even though the flow is longer, there is
less sediment available to be transported shoreward. The offshore phase, on the
other hand, is much faster, so colossal amounts of sediment are scooped up from
the sea bed, which then get transported rapidly seaward and dumped offshore
somewhere.

If you’ve managed to follow me up to this
point, you’re probably beginning to slide down the slippery slope of idealized
models and theoretical thinking, which means you’ll probably think both of
those ideas fairly reasonable. In fact, like most things related to
infragravity waves, they are both really just hypotheses, and a subject of ongoing
research.

Lastly, some of live in places where the
waves sometimes become so huge and out of control that you have to start
looking for smaller spots to surf. Usually these are places facing away from
the main swell direction, behind a headland or even a short distance up an
estuary. In my experience, these spots seem to be much more dangerous than
those on the open coastline, even though they pick up a lot less swell. They
nearly always have stronger rips, more water moving and give you a lot more grief
getting in and out of the water than their open-ocean counterparts. But
why?

It is because of the infragravity waves.
Even if the ordinary waves are filtered down to half their size by the effect
of a headland, an estuary or some other feature, the infragravity waves just
keep ploughing on through, actually getting bigger
as they pour into the nooks and crannies of tucked-away pocket beaches. The
infragravity waves corresponding to a 30-foot swell will still reach that
round-the-corner spot even though the ordinary waves might have be filtered
down to a manageable six foot. It is the disproportionately large infragravity
waves that cause all that extra water moving, and all those rips and surges.
Even if you think you’ve escaped the gigantic swells, the infragravity waves
are still there, lurking underneath, ready to spoil your surf session or,
worse, sweep you out to sea.